| Junggarsuchus | |
|---|---|
 | |
| Holotype skull (IVPP V14010),Paleozoological Museum of China | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Archosauria |
| Clade: | Pseudosuchia |
| Clade: | Crocodylomorpha |
| Clade: | Solidocrania |
| Genus: | †Junggarsuchus Clarket al.,2004 |
| Type species | |
| †Junggarsuchus sloani Clarket al., 2004 | |
Junggarsuchus (/ˌdʒəŋɡərˈsukəs/) is anextinctgenus ofsphenosuchiancrocodylomorph from theMiddle[2] orLate Jurassic[1][3] period ofChina. Thetype and only species isJ. sloani. Thegeneric name ofJunggarsuchus comes from theJunggar Basin (theanglicization ofDzungar),[4] where the fossil was found, and theGreek word "souchos" meaningcrocodile. The specific name, "sloani" is in honor of C. Sloan, who is credited with finding theholotype.[2]
Junggarsuchus was found in the upper part of the Lower Member of theShishugou Formation inXinjiang,China at the Wucaiwan locality.[2][5] The type and only specimen was described in 2004 by James Clark,Xu Xing, Catherine Forester, and Yuan Wang inNature,[2] but it did not receive a full osteological description until 2022 when Alexander Ruebenstahl, Michael Klein, and Yi Hongyu published a monograph along with two of the original describers James Clark and Xu Xing.[3]
It is a relatively small animal, with the skull of the holotype only measuring 144 millimetres (5.7 in) in length. The right forelimb was about 29 centimetres (11 in) long from the shoulder to themetacarpals.[3] This would make the animal in life only about the size of adomestic cat. However, its overall length is unknown because the holotype only preserves a single vertebra from the tail.[2]
The holotype, given the designationIVPP 14010,[2] consists of an almost complete skull with an intact brain-case and lower jaws, most of the left forelimb, the proximal ends of the ulna and radius of the right forelimb along with the right humerus, fifteen cervical and dorsal vertebrae along with most of the ribcage, and an associated caudal vertebra.[3] Most of the skeleton was fully articulated, with some elements being found in association and disarticulated.[2] More of the specimen had yet to be prepared at the time of the monograph's publication, and some of the bones remained obscured by the matrix at the time of its re-description.[3]
The skull of the holotype was transported from theInstitute of Vertebrate Paleontology and Paleoanthropology, where it was initially reposited, toGeorge Washington University, where it was studied and comprehensively re-described by Ruebenstahl and colleagues using modernCT imaging technology. The preparation of the skeleton was so exquisite that elements of the skeleton which had been damaged during fossilization were able to be glued together by the preparators with very little unconformity.[3]
In their initial description of the skull, Clark and colleagues noted that there was very littlecranial kinesis and there were attachment sites for very powerful jaw muscles, which are derived traits found in modern crocodilians.[2] With regard to the post-cranial skeleton, they noted that the spine of the holotype probably had very low vertical mobility across its length and it was mostly adapted for lateral motion, consistent with modern crocodilians.[3][2] However, a very significant difference betweenJunggarsuchus and its modern relatives was the complete lack ofosteoderms in the specimen, despite the lack of spinal mobility, when these traits were presumed to have co-evolved. They suggest that the lack of preserved osteoderms may be a product of thetaphonomic conditions of the specimen, or that it may have been a juvenile when it died.[2]
As in other sphenosuchians, the limbs ofJunggarsuchus were adapted to terrestrial locomotion (movement on land), rather than the semi-aquatic locomotion seen in living crocodilians.[3] These adaptations include: a vertically orientatedupper arm bone, ball-and-socket shoulder joint, and a functionally tridactyl (three-fingered) hand due to a reduction of digit 5 and the absence of digit 1.[2] However, the fact that these terrestrial adaptations appear to be common toJunggarsuchus and to more basal animals likeSaltoposuchus, means that this probably represents theancestral condition incrocodylomorphs, rather than aderived trait of "sphenosuchians", and the semi-aquatic adaptations of the crown group only appeared much later in their evolution.[3][6]
The specializations observed in the skull ofJunggarsuchus are primarily related to the reinforcement of the skull's structure and the development of stronger jaw musculature.Diapsid skulls are ancestrally relatively lightweight, and so to deliver high bite forces without damaging the skull requires significant specializations.[7] This can be seen in the dinosaurTyrannosaurus, which has a completely fused nasal bone (the bone on the dorsal midline of the snout), whereas most othertheropods have sutures in the topology of the bone in that location.[8] Several of the skull's fenestrae have been reduced in size compared to other early crocodylomorphs, most notably theantorbital fenestra and thesupratemporal fenestra.[2] In addition, several of the bones of the palate have become enlarged when compared with more basalcrocodylomorphs. Thequadrate bone is also much closer to thelaterosphenoid bone and there is no suture between theparietal bones.[3] Both of these reduce the flexibility of the skull, which has been suggested to enable greater bite forces to be introduced without damaging the skull.[2][9] However, this trend of an increasing robustness of the skull is not uniform, and two notable adaptations which buck this trend are the appearance of an additional fenestra in the quadrate and a lack of significant contact between thenasal andlacrimal bones. While most of the skull characteristics demonstrate the intermediate placement ofJunggarsuchus on the family tree of crocodylomorphs, there are several entirely novel adaptations that the skull shows. The lateral surface of theangular bone has extensive attachment sites for jaw musculature, which is seen in some crown crocodilians, but not in other intermediate forms. This, like several other adaptations, most likely aided in delivering higher bite forces to potential prey items.[3]
Another notable trait of the skull ofJunggarsuchus is the appearance of pneumatic spaces in the quadrate, theparabasisphenoid, and possibly thepterygoid. While this seemingly contradicts the trend of skull solidification inJunggarsuchus and related taxa (basal solidocranians), Ruebenstahl and colleagues suggest that it is possible that these pneumatic spaces enabled the tissues of the skull to withstand and absorb shocks that would otherwise damage a structure made of solid bone.[9] However, the authors note that this is only inferred, and the evolution of cranial pneumaticity in crocodylomorphs may instead reflect the invasion of these tissues by cranialsinuses.[3] Poor preservation of the pterygoid of the holotype also makes it difficult to infer the exact degree of pneumaticity or to speculate with regard to any of its possible functions.[3]
Notably, theinner ear anatomy ofJunggarsuchus shows significant terrestrial adaptations. Thesemicircular canal is tall and narrow, unlike in aquatic crocodilians, which is believed to have aided the animal in the orientation of the head and the gaze. This would have been necessary to hunt terrestrial prey and to maintain balance while moving on land.[3]
Despite the exceptional preservation of the skull, there are a few key areas where the arrangement of the skull bones is uncertain due to deformation or damage that the skull suffered during thefossilization process. These areas are the contact between theangular andsurangular bones and the contact between thesquamosal andpostorbital bones. Based on CT data, Ruebenstahl and colleagues suggest that it is likely that the surangular is short in the jaw ofJunggarsuchus and does not have significant contact with the angular. However, it is possible that the surangular has significant contact across a significant length of both thearticular and angular bones, which would make it similar to the condition of its close relativeDibothrosuchus.
The other uncertainty is in regard to the contacts between the quadrate and the postorbital bones in the rear of the skull. Breaks in the bone as it was being preserved make it difficult to distinguish which fractures are reflective of taphonomic damage and which ones are reflective of actual sutures on the bone. The authors hypothesize that the postorbital only contributes to the dorsal ridge of the supratemporal fenestra, although they consider the possibility that the postorbital extends down the posterior ridge of the fenestra, running parallel to the squamosal rather than meeting it straightforwardly.[3]
Junggarsuchus displays numerous adaptations forcursoriality.[3] These include the reduction of the number ofdigits in contact with the ground from five, which is the ancestral condition inpseudosuchians, to only three in the forelimbs ofJunggarsuchus.[2] The reduction of the outer digits such that they are not in contact with the ground has been previously recognized to be a trend conserved among cursorial tetrapods,[3][10] and digits one and five are very reduced inJunggarsuchus.[2] Other limb specializations include an enlarged surface on the anterior edge of thescapula, which has been interpreted as a site for expanded muscle attachments to aid in limb retraction. The joints of the shoulders and limb bones also appear to be generally oriented in such a way that the limbs would be held directly underneath the body and would have the ability to flex underneath the body of the animal.[3] This distal bones of the arm (theulna andradius) are also much longer relative to thehumerus than in other archosaurs (though they are still not as long as the humerus). This has been interpreted as an additional indication of cursoriality in other crocodylomorphs.[11] The hind limbs are not preserved in the holotype,[2] so it is not certain if thefemur,tibia, andfibula also exhibit this relationship.[3]
The notable lack ofosteoderms which, in the original description, was suggested to be purely taphonomic,[2] is suggested by later authors to be reflective of a genuine lack of these features as part of a general trend towards more terrestrial mobility and flexibility which is not seen in the modern relatives ofJunggarsuchus. Characteristics of the spine including thezygapophyses are suggestive of a much more flexible overall range of motion that most modern crocodiles lack completely.[3] This level of terrestrial flexibility is common in early-diverging crocodylomorphs as well asarchosaurs in general. Most of the close relatives ofJunggarsuchus suich asDibothrosuchus,[12]Terrestrisuchus,[13] andDromicosuchus[14] are similarly gracile and adapted for fast terrestrial movement. This suggests that the aquatic adaptations seen in modern crocodiles, as well as in many of their extinct relatives, like thethalattosuchians, are not the ancestral condition of this group.[3]
Nothing is known about the hind limbs, hips, or tail ofJunggarsuchus because the holotype, which is the only known specimen, does not preserve these elements.[2] However, there may be elements of the holotype which have yet to be prepared which could reveal information about these areas of the skeleton once the rock has been removed and any new bones have been described.[3]
The original description ofJunggarsuchus by James A. Clark and colleagues found it to be the sister taxon to the cladeCrocodyliformes and also found that the traditional grouping,Sphenosuchia, was not amonophyletic group, but rather formed a largepolytomy with a grade basal to crocodyliformes.Synapomorphies of theJunggarsuchus-crocodyliformes clade in their analysis included:exoccipitals that meet on the midline above theforamen magnum, a large extension on the bottom sides of the exoccipitals which contacts thequadrate,jugal bones which are strongly arched, a narrowing of the rear portion of theparietal bone, and afenestra in thequadrate bone. Their analysis is shown below.[2]
In 2017, Juan Martin Leardi and colleagues redescribed the closely related taxon,Macelognathus which had originally been described byO. C. Marsh in 1884 as a species of dinosaur.[15] They used the same data set as Clark and colleagues, but their analysis included many more taxa including the recently described taxaAlmadasuchus andCarnufex. They recover a slightly more well-resolved phylogenetic tree, with the exception of a polytomy at the base. Their analysis is notable for includingJunggarsuchus as being slightly more basal than a monophyleticHallopodidae. Synapomorphies of the clade includingJunggarsuchus, Hallopodidae, and crocodyliformes include the following: an expandedbasisphenoid bone, exoccipital bones which contact the quadrate, an enclosed passage for theinternal carotid arteries, a large post-temporal fenestra enclosed by thesquamosal and the exoccipital bones, and a radius which is shorter than the humerus. They also recoverMacelognathus as a close relative ofHallopus.[16] Other authors have suggested that this is difficult to corroborate or even disputed this result becauseMacelognathus is known mostly from skull material andHallopus does not have any of its skull preserved.[3][15] The results of their analysis are shown below.[16]
| Crocodylomorpha |
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In their re-description, Ruebenstahl and colleagues recovered a new clade,Solidocrania, meaning "solid skulls", in reference to the lack of cranial kinesis.[2][3] This clade was defined at the least inclusive clade which containsJunggarsuchus,Macelognathus, andAlmadasuchus. The results of their phylogeny showed these three taxa to form a grade basal to Crocodyliformes, and thus Crocodyliformes itself would be included within Solidocrania.[3] Synapomorphies of this clade were reported to be: two largepalpebral bones, asquamosal bone which contacts the rear surface of thequadrate bone, an enclosure of the cranioquadrate canal by the squamosal and occipital bones, a convergence of the laterosphenoid and the quadrate, a contact between the otoccipital and the quadrate, an expansive and pneumatic parabasisphenoid, a developed ridge on the back of the ectopterygoid projecting along the interior surface of thejugal bone, a front edge of thescapular blade which is larger than the rear edge, and a lowolecranon process of theulna.[3] Additional synapomorphies were suggested, but these were more weakly supported. The results of their phylogenetic analysis, calculated by finding a strict consensus of the two most parsimonious trees, are shown below.[3]
Novel results of this phylogeny included findingHallopodidae to bepolyphyletic as well as the taxonCalsoyasuchus to be the sister taxon ofThalattosuchia, when it has traditionally been considered to be agoniopholid.Phyllodontosuchus was also found to be the sister taxon toJunggarsuchus,[3] although this was weakly supported becausePhyllodontosuchus is only known from a single, heavily deformed skull and associated teeth,[17] and its placement in the authors' phylogeny varied considerably in regard to its affinities.[3] The authors also noted thatHsisosuchus, generally considered to be basal to theziphosuchian-neosuchian split, may actually be more closely related to ziphosuchians.[3]
The three taxa that define Solidocrania,Junggarsuchus,Macelognathus, andAlmadasuchus, are allLate Jurassic in age, yet they are all believed to be more basal thanCrocodyliformes, which are known to have originated in theLate Triassic.[2][3][18] Ruebenstahl and colleagues state that this finding implies that there is a 50 million-year-longghost lineage of solidocranian taxa that stretches back into the Triassic. If their hypothesis is correct, this would mean thatJunggarsuchus and its relatives represent some of the last surviving non-crocodyliform crocodylomorphs. However, the authors are careful to note that, given the late appearance ofJunggarsuchus and other basal solidocranian taxa, it is possible that Solidocrania is not a natural clade and that the uniting skull characteristics could be a secondarily derived trait that is a result ofconvergent evolution rather than traits inherited from a shared common ancestor with crocodyliformes.[3]
In 2023, Emily Lessner, Kathleen Dollman, James Clark, Xu Xing, and Casey Holliday performed an analysis ofpseudosuchian facial nerves using skull material from over 20 different taxa includingJunggarsuchus. One of their findings was thatJunggarsuchus was among the earliest-diverging crocodylomorphs to have a linear arrangement offoramina below the teeth of thelower jaw. All of the earlier-diverging taxa had these foramina arranged randomly.[19]
Their analysis concluded by noticing a marked trend in the tactile sensitivity of pseudosuchian snouts as they evolved, with successively more derived groups possessing increasing density of nerves along the snout. These nerves are used to detect motion in the water by modern crocodilians, but the study concluded that the increase in these nerves predates the evolution ofsemiaquatic crocodyliformes. One possible explanation for this apparent discrepancy that the authors suggest is that earlier-diverging terrestrial crocodylomorphs may have exhibited a feedingecology that included foraging on or near the ground for prey.Junggarsuchus is a notable outlier in this trend because the inferred density of these nerves is much lower than in comparable taxa such asMacelognathus andLitargosuchus. However, the authors do not state any possible implications this may have for the feeding ecology ofJunggarsuchus specifically.[19]
Junggarsuchus was almost certainly carnivorous, like most other crocodylomorphs.[3] It possessedxiphodont teeth withserrations which were adapted for cutting through flesh.[2] However, this presumption is complicated by the closely related taxonPhyllodontosuchus,[3] which possessedheterodont teeth.[11] Heterodonty is an adaptation that is uncommon in reptiles, and when it has appeared (e.g. inPakasuchus andChimaerasuchus), it has been assumed to be an adaptation to novel feeding strategies.[20][21][22] Similarly, although not as strikingly, the teeth ofMacelognathus are non-serrated on the crowns and theirmandibular symphysis is entirely toothless, which has been interpreted as an adaptation for herbivory.[11]Junggarsuchus does not share any such adaptations; it has more traditionally serrated teeth which extend all the way to the end of thedentary andpremaxilla.[22] The reason for this dramatic variation in these closely related taxa is uncertain, and may be reflective of the scarcity and relative incompleteness of the remains of these taxa, which only allows for limited comparisons to be drawn between them.[15]
There is no direct evidence to indicate exactly what the diet ofJunggarsuchus may have consisted of, but given its size and dentition, most authors have stated that the most reasonable assumption is that it was a pursuit predator of small vertebrate prey.[3][11] Furthermore, the overall shape of the skull and the ratio of its height to width (i.e. its "flatness") has been shown to be more similar to modern crocodilians than it is to contemporary crocodylomorphs. This is related to muscle attachment sites for themedial pterygoid muscle, which is an important muscle used in closing the jaws, which meant thatJunggarsuchus may have been adapted to catching small prey.[23] In total, the indirect evidence seems to indicate theJunggarsuchus most likely fed on small animals likeprimitive mammals,squamates, and possibly hatchlingdinosaurs.[11]
The only remains ofJunggarsuchus so far described were discovered near the town of Wucaiwan inXinjiang,China.[5] This locality is a part of the lower member of theShishugou Formation,[24] which ranges from 164 to 159 million years ago. This interval spans the transition from theMiddle Jurassic to theLate Jurassic, though most of it has been recently dated to the Late Jurassic.[25] This region is inland and arid today, but in the Late Jurassic, it formed a coastal basin on the northern shores of theTethys Ocean.[26]
The lower (or Wucaiwan) member of the Shishugou consists primarily of redmudstone andsandstone deposits. This is interpreted to have consisted of a woodedalluvial fan environment which experienced periodic flooding, which accounts for the wide variety of small-bodied animal fossils preserved in the area as well as the abundance of fossilized trees. The Wucaiwan member preserves fossils oflungfish,amphibians,crocodilians,tritylodonts, anddinosaurs of various sizes. However, the upper portions of this member, whereJunggarsuchus was found, are believed to have consisted of more traditionalfluvial orwetland environments with less-intense flooding than the lower portions of the member.[25] The climate of the area during the Late Jurassic was temperate and seasonally wet and dry.[26] This pattern of rainfall led to the prominence of seasonal mires, possibly exacerbated by substrate liquefaction by the footfalls of massivesauropods which created "death pits" that trapped and buried small animals.[25][27]
There have also been significantvolcanic ash deposits found in the Wucaiwan member, indicating that volcanic activity in the western part of China was increasing at this time.[25]
A variety of small animals have been uncovered from theShishugou Formation. Various remains of small animals have been referred to various groups but have yet to be givenbinomial names. These include remains oflungfish,brachyopoid amphibians,docodont andtritylodontmammaliamorphs,lizards, andturtles. Some of these are preserved almost completely and in articulation.[25] There is also a small crocodylomorph which may be related toJunggarsuchus that has yet to receive a formal description or name.[28][29] Various dinosaur remains that have not yet been named have also been recovered from the area. These includestegosaurs,ankylosaurs,ornithopods,tetanurans, and a putativeornithomimosaur.[24][30]
Named fossils include the primitive mammal-relativeYuanotherium, thecrocodyliformesSunosuchus andNominosuchus, and the pterosaursSericipterus andKryptodrakon.[25] Dinosaurs are the most common and diverse part of the terrestrial fauna found in the Shishugou.[26] They are represented by smallornithischians such asYinlong,Hualianceratops, and "Eugongbusaurus" as well as by thesauropodsKlamelisaurus,Bellusaurus, andMamenchisaurus sinocanadorum. All large terrestrial predators in the ecosystem weretheropods. These ranged from smallcoelurosaurs likeHaplocheirus,Aorun, andGuanlong to largecarnosaurs likeSinraptor. Also notable in the area was the smallceratosaurLimusaurus, which was preserved in one of the muddy "death pits".[25]
Also known as Wucaiwan
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